"Apply a very thin layer of agar before you inoculate the plate, just enough agar so the specimen can grow out. Then as soon as you see mycelium from the mushroom, apply another layer of very hot agar over the whole works, covering the growth and first layer completely. Contaminates and all. The mycelium from the mushroom will crawl through to the top layer, leaving behind the contaminates. You'll need to transfer the new growth to a new plate. It doesn't hurt to do the exact same thing with the new transfer either. Give the transfer a couple of days to start growing and then cover it with hot agar again immediately. Just let the plate colonize completely for use. The mushroom mycelium will bubble up from the top layer of hot agar that's applied, leaving you to think that you've zapped and killed the mushroom mycelium. Don't worry, you haven't."